the protection of the anchorage, the number of square acres enclosed. If there be any backwater, state its extent, how the scouring, if any, is managed, at what time of tide, and what is its apparent effect; and at all places wherever backwater is used, it may be as well to sound off the mouth of the port to as great a distance as the effect of the scouring action can possibly extend, for occasionally injurious effects have been produced by this powerful agent at a distance scarcely contemplated. State all deposits, siltings up, and at what rate they proceed.

Foreign Ports.

35. In visiting foreign ports, a particular account should be given of the resources of the place in the event of vessels requiring either a repair or a refit. Such as

whether there are any docks, wet or dry? what sized vessels they are capable of receiving, and how many at a time? is there a patent slip or gridiron ? &c. How near vessels, of particular dimensions, can approach the wharfs, or at what time of tide lie alongside of them; whether there are sheers for removing masts, and of what size, or cranes for lifting machinery and boilers; whether there be a dockyard or arsenal, or whether stores can be procured from other sources? Whether there is a steam-yard, and to what extent they cast and manufacture machinery or boilers, or can repair steamers?

Whether there is a coal-depôt, and what quantity of coal can be generally relied upon as at hand; the nature and quality of the material, &c.

Are there any piers, jetties, or wharfs for landing passengers, or cranes for carriages, and at what time of tide available? If the country be low, are there any sea walls, and would the country be flooded by their removal?

Waves.

36. Lastly, the attention of the observer should be directed to the measurement of the height, the extent, and the velocity of the waves of the ocean;-not only of those

E

high swelling seas which are common to every gale, but especially of those gigantic ridges which are occasionally met with off Cape Horn, the Cape of Good Hope, and even in the Atlantic, coming in couplets and triplets in the course of a gale, and occasioning fearful lurches which are long remembered. Opinions differ greatly as to the dimensions of these stupendous bodies, and any observations which will assist in determining their limits cannot fail to be acceptable. The inquiry is, first, as to the height of the solid wave above the mean water-level. Secondly, the distance of the ridges apart. Thirdly, the rate at which the wave travels, and whether the height and distance of the ridges vary with the velocity. Fourthly, what is the greatest estimated extent of any one of those ridges.

The most simple way of measuring the height is, when the vessel is in the lowest part of the trough between two following seas, to ascend the rigging to such a height as will bring the top of the wave on with the horizon, to put a mark, note the inclination of the vessel, and at leisure to measure the perpendicular height of the eye above the water-line, which we may presume will be double the height of the wave above the mean water-level. It will necessarily require several observations to be made before any satisfactory conclusion can be arrived at. The distance of the waves apart may possibly be tested by actual measurement, by means of the lead-line and a float veered out to such a distance that the float shall be on the crest of one wave when the ship is on the top of the other; and the rate may be determined by the time occupied by the wave in passing from the float to the ship: the rate of the ship through the water, and the angle her course makes with the route of the wave being known. There are other methods of solving this interesting problem which will, no doubt, occur to the intelligent observer, and they are sufficiently numerous to afford ample exercise of his ingenuity, but all are attended with difficulty, owing to the circumstances under which the observations are required to be made.

Remarks upon the Currents which prevailed in the passage across the Atlantic.

"From the time we quitted Teneriffe, with the N.E. trade wind, until we lost the breeze in lat. 7° 40' N., long. 26° 40' W., the current set on an
average S. 540 W. true, at the rate of 11 miles per day. On losing the trade and entering the calm latitudes, the westerly current ceased, and the
next 24 hours the ship was set N. 830 E. true 23 miles. The meeting of the opposite currents was marked by a strong ripple, which was traced to a
considerable distance. The four succeeding days, in which we changed our position from lat. 7° 20' N., and long. 266 58′ W., to lat. 3° 56' N.,
long. 26° 44' W., the current ran S. 700 E. true 13 miles per day. Here we met the S.E. trade, and with it experienced a strong current, which
carried us N. 620 W. true 23 miles per day, until we made Fernando Noronha. Hence to a position 100 miles due E. of Cape Ledo the current set
between S. 780 W. and S. 210 W. (true) on an average daily rate of 27 miles," &c.

"While in Rio Janeiro H.M. ships A. and B., the packet C., and a fast-sailing schooner the D. arrived, and we learnt that the A. had crossed the
equator in 180 W., the B. in 250 W., the C. in 2940 W., the D. in 390 W., whilst we crossed in the E. in 300 W.; and upon inquiry it appeared that
the passages from England were as follows, viz.: the A. was 49 days, B. 40 days, C. 38 days, the E. 36 days, and the D. 110 days, having got so far
to the westward that she could not weather Cape St. Roque, and was obliged to stand back to the variable winds to regain her easting. Thus it
appears that, with the exception of the D., the passages were shortened in proportion as the equator was crossed to the westward," &c. &c.

APPENDIX No. 2.

TO FIND THE HEIGHT OF AN OBJECT THE DISTANCE OF WHICH IS KNOWN.

RULE. To the observed altitude apply the true dip, less the terrestrial refraction; * the result call corrected altitude: to the log. of the distance in yards add the constant 8:073007, and find the log. of the sum, which turn into arc, and add to the corrected altitude; then to the log. tangent of this sum add the log. of the distance in yards, as above mentioned: the result will be the log. of the height of the object in yards.

EXAMPLE.-Mount Etna was seen at 57 nautical miles, or 115,650 yards, distance, and subtended an angle of 1° 30′ 00′′ with the horizon; elevation of the eye 20 feet; required the height of the mountain?

* The terrestrial refraction varies from to part of the dip.

If the Dip Sector had been used the observed Dip should be substituted for these quantities.

APPENDIX No. 3.

TO FIND THE Constant for A HEIGHT, IN ORDER TO COMPUTE ITS DISTANCE READILY FROM ITS OBSERVED ALTITUDE.

RULE. From the log. of the height in yards subtract the constant log. 6-5424481, halve the sum, find its sine, and take out the corresponding co-sine, which is the constant required (a).

To find the Distance.

RULE. From the observed altitude subtract the dip, less the terrestrial refraction,* and call the remainder corrected altitude. To the constant above mentioned 6-5424481 add the cosine of the corrected altitude, and from the cosine of the sum subtract the corrected altitude, The remainder is the log. of the approximate distance in arc. Divide the approximate distance so found by the proportion of terrestrial refraction allowed, and subtract the quotient from the before found corrected altitude for the true altitude.

Lastly, add the cosine of the true altitude to the constant due to the height of the object (a); find the cosine of the sum, and subtract from it the true altitude; the remainder is the distance in arc required.

EXAMPLE.-Observed the altitude of Snowdon to be, On 45 00

its height being 3543 feet

required its Constant and its Distance, height of eye

being 14 feet.

1181 yards

Off 45 10

45 5 mean.

35 38 35.6 = distance of object.

* The terrestrial refraction varies from to of the dip.

41 42

3

25

38 17